The invention described herein relates to nuclear reactor fuel assemblies and more particularly to an improved grid used in holding fuel rods and control rod guide tubes in spaced relationship with each other in a fuel assembly.
Many different designs of grids are used for holding fuel rods and control rod guide tubes in spaced relationship with each other in a nuclear reactor fuel assembly. One well known design disclosed in U.S. Pat. No. Re. 28,079 includes interleaved grid straps which form multiple cells arranged to contain the fuel rods and guide tubes. The fuel rod in each cell is supported by springs on two adjacent straps which project inwardly and contact two sides of the fuel rods. Each of the other two adjacent straps contain a pair of vertically spaced dimples which contact the other two sides of the fuel rod thus providing a six point support to the rod. This design provides many benefits, such as resiliently supporting fuel rods in each egg-crate cell and acting to accurately space the fuel rods while minimizing fuel rod vibration and permitting fuel rod thermal expansion during reactor operation.
The disadvantages however are that the dimples occasionally are torn from the metal surface during the time a fuel rod is being loaded into the grid and since the fuel rod is then not properly supported, the grid must be removed from the fuel assembly being assembled. Further, special dies are required to punch out the springs and dimples with a high degree of precision and accuracy to assure that coolant flow distribution along the fuel rods and through the grids will be uniform and constant. Manufacturing labor costs are therefore relatively high. Also, since the grids usually are made of Inconel, which work hardens very quickly, cracks occasionally appear at the point where the dimples are punched outwardly from the stock material. Since extremely high standards and tolerances are set for the manufacture and quality of grid components, small defects which would not affect reactor performance often require scrapping of the grid material.